As the structures making up functional units across several industries now approach the nanoscale (<100 nm), metrology of these structures becomes ever more challenging. To resolve structures at this scale commonly electron microscopy is employed as it offers an image resolution below 1 nm for some scanning electron microscopes (SEMs) and below 0.1 nm for some transmission electron microscopes (TEMs). In order to prepare SEM and TEM samples of specific structures and devices, the capability to image and cut with very high precision is crucial. The cutting can be performed either by employing ultra-precise blades in an ultra-microtome or by using finely focused beams of charged ions in so called focused ion beam microscopes (FIBs). Machines combining the capabilities of FIBs and SEMs, commonly referred to as FIB-SEMs as well as ultra-microtomes are available on the market.
When investigating structures with at least one dimension <200 nm it is often advantageous to prepare so called ‘lamellas’ containing a cross-section of the structures of interest. Ideally, the lamella thickness is of a similar dimension to the structures of interest. The lamellas are cut from an original sample. Depending on the application and the technique used for analysis, the lamella may remain attached to the original sample or be removed from the sample using a manipulator. Different types of manipulators are available to lift the sample out in the microscope chamber (in situ lift-out) or outside the chamber under an optical microscope (ex situ lift-out). Once the lamella is attached to the manipulator needle it can either be analysed directly on the needle or be transferred onto a holder for further analysis. In the case of an in situ manipulator in a FIB or FIB-SEM the lamella can be thinned further on the needle or once it has been transferred onto a suitable substrate. Equipment and techniques are also available to further thin samples ex situ.
Independent of the exact lamella geometry, the lamella thickness is a vital parameter as it strongly influences the quality of the analysis performed on the lamella. Methods to measure the lamella thickness exist in the prior art:
(1) The thickness can be measured approximately by imaging the lamella edge on. However, this assumes that the lamella has parallel side-walls which is not usually the case.
(2) A STEM detector can be used to measure the intensity of an electron beam (as in US2012/0187285) transmitted through the lamella and comparing this intensity to a calibration curve obtained from the same material.
(3) The intensity of a beam of transmitted light can be measured using a photodetector. The lamella thickness can be determined by comparing the light intensity to a calibration curve obtained from the same material (as in our earlier patent U.S. Pat. No. 8,227,781).(4) The thickness can be determined by comparing the intensity of a beam of transmitted He-ions detected using a secondary electron detector to a calibration curve from the same material (see ‘In-situ Thickness Assessment during Ion Milling of a Free-Standing Membrane Using Transmission Helium Ion Microscopy’, Microscopy and Microanalysis Epub Apr. 29, 2013).(5) Once the lamella is transferred into a TEM, the thickness can be measured using Electron Energy Loss Spectroscopy (see Iakoubovski et al., ‘Thickness Measurements With Electron Energy Loss Spectroscopy’, Micr. Res. Tech 71, p. 626 (2008)).
All the present methods have clear disadvantages. Method (1) is approximate and not suitable for lamellas where the thickness varies. Variation is common due to differential material milling rates, alignment issues or redeposition in the case of FIB preparation. Method (2) to (4) require pre-existing calibration curves on the same material as the sample being prepared. These calibration curves are not straightforward to obtain. Method (5) requires the removal of the lamella from the FIB or SEM where it is being prepared and measurement in a TEM which is time consuming.
Also, when preparing lamellas with a FIB or a FIB-SEM, implantation of ions into the sidewalls of the lamella occurs. The implanted layer is usually damaged (“amorphised”) by the milling process and therefore has to be removed before the analysis. This can be done by using very low energy ion milling in the FIB or by removing the lamella from the FIB and using other techniques like low angle argon ion milling (see J. Mayer et al, 2007, MRS bulletin, vol. 32 400-407). However, the exact thickness of the amorphous layer is difficult to determine with established techniques and therefore usually literature values obtained using 30 kV gallium ions milling silicon are used. This causes issues of either removing too much material and thereby accidentally removing part of the intact lamella or not removing enough material which results in poor image quality during analysis of the lamella in an SEM or TEM.
Despite the various known techniques there exists a need for a practical technique which addresses the problems known in the prior art. It is in this context that the present invention has been devised.